Since the earliest days of mobile police, fire, and emergency medical services, mechanical sirens have helped to clear the way producing a particular whoo—whoo sound distinctly different from the bells, horns, and whistles of the other vehicles.
The present invention makes the unique sound by rapidly momentarily turning on-off-on-off the air flow of its centrifugal air pump. These sirens were originally hand cranked, later driven by friction wheels against other rotating machinery, and still later by their own electric motor. In his 1925 U.S. Pat. No. 1,566,761 Miles disclosed an open rotor with straight pumping vanes radiating from the center with small right angle end flanges for closing the stator ports. In a subsequent U.S. Pat. No. 1,739,727 Miles demonstrates curved vanes and confirms that “as the air is forced outwardly and the ports are intermittently opened and closed by the rotor flanges a loud noise will be produced.”
Refinement of the siren during the 30's and 40's brought better rotor shapes for improved air flow, enclosed rotors for less resistance, and rotor clutches for coasting. The motor driven electromechanical sirens of the 60's were producing 120+decibels of square wave form sound from 10 inch diameter 37 pound machines drawing over 300 starting amps and 175 running amps of 12 volt power.
By the 70's increasing demand for electrical power in emergency equipment brought on by more warning lights, communication radios, and computers prompted the industry wide switch to electronic sirens requiring only 15 to 20 amps of power. These sirens mimic the whoo—whoo sound electronically with transistors and then project it from speakers. This sound is in a sine wave form, much like the ripples on the lake from where a rock was tossed.
During the last 30 years improvements in automobile insulation and soundproofing are rendering the electronic siren ineffective. It is not uncommon for the Fire Chief to have to climb down out of his fire truck and walk ahead to a stopped motorist so he can tap on their window in order to get their attention and ask them to pull their vehicle to the right.
However, the motor driven mechanical siren of this application with its square form sound wave penetrates through a closed modern vehicle, even with the air conditioning and the radio on, to alert the driver of an approaching emergency vehicle.
Therefore, there is a strong desire for a new mechanical siren which is only 5 inches in diameter and yet able to deliver an appropriate 123 decibels of sound while drawing only 28 amps of power.